The “flame spray” or “thermal spray” process has been well documented and described in the prior literature. As described in U.S. Pat. No. 6,001,426: “Thermal spraying is a process of applying coatings of high performance materials, such as metal, alloys, ceramics and carbides, onto more easily worked and cheaper base materials. The purpose of the coating is to provide enhanced surface properties to the cheaper bulk material of which the part is made.” As also stated in the same patent: “Thermal spray includes a variety of approaches, but can be grouped into three main coating processes: combustion, wire-arc, and plasma.” Such thermal spray processes can be further subdivided into continuous and detonation processes.
All of these known thermal spraying processes have one thing in common: they all use an external energy source to provide the heat to soften or melt the material that is to be sprayed. In addition, the rate of deposition of these thermal spraying processes is relatively low and there is a need for higher spray rates.
The traditional flame spray processes use either a gas fuel (hydrogen) and oxygen mixture for the heat source or a high-powered electric arc. The hydrogen-oxygen heat source requires large high-pressure tanks of both gases, while the electric arc typically requires 55 Kilowatts of electric power (Sulzermetco F4 Gun Series).
One of the problems with the present thermal spraying process is the difficulty of controlling the chemical environment and preventing oxidation reactions which can occur on the surface of the powder particles prior to their impingement on the substrate.
It will be helpful to describe the present types of flame spray processes. These descriptions are available on the web site of the Gordon England Company in the UK, www.gordonengland.co.uk.
Combustion Powder Thermal Spray Process:
This process, also called the Low Velocity Oxygen Fuel Process (LVOF), is basically the spraying of molten material onto a surface to provide a coating. Material in powder form is melted in a flame (oxy-acetylene or hydrogen most common) to form a fine spray. When the spray contacts the prepared surface of the substrate material, the fine molten droplets rapidly solidity forming a coating.
The main advantage of this flame spray process over the similar Combustion wire spray process is that a much wider range of materials can be easily processed into powder form giving a larger choice of coatings. The flame spray process is only limited by materials with higher melting temperatures than the flame can provide or if the material decomposes on heating.
Combustion Wire Thermal Spray Process (Metal Spraying):
This flame spray process is basically the spraying of molten metal onto a surface to provide a coating. Material in wire form is melted in a flame (oxy-acetylene flame most common) and atomized using compressed air to form a fine spray. When the spray contacts the prepared surface of a substrate material, the fine molten droplets rapidly solidify forming a coating.
This flame spray process has been extensively used in the past and today for machine element work and anti-corrosion coatings.
Plasma Spray Process:
The Plasma Spray Process is basically the spraying of molten or heat softened material onto a surface to provide a coating. Material in the form of powder is injected into a very high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. The hot material impacts on the substrate surface and rapidly cools forming a coating.
The plasma spray gun comprises a copper anode and tungsten cathode, both of which are water cooled. Plasma gas (argon, nitrogen, hydrogen, helium) flows around the cathode and through the anode which is shaped as a constricting nozzle. The plasma is initiated by a high voltage discharge which causes localized ionization and a conductive path for a DC arm to form between the cathode and anode. The resistance heating from the arc causes the gas to reach extreme temperature, dissociate and ionize to form a plasma. The plasma exits the anode nozzle as a free or neutral plasma flame (plasma which does not carry electric current) which is quite different from the Plasma Transferred Arc coating process where the arc extends to the surface to be coated. Powder is fed into the plasma flame most commonly via an external powder port mounted near the anode nozzle exit.
Plasma spraying has the advantage over combustion processes in that plasma spraying can spray very high melting point materials such as refractory metals like tungsten and ceramics like zirconia. Plasma sprayed coatings are generally much denser, stronger and cleaner than other thermal spray processed with the exception of HVOF and detonation processes.
Disadvantages of the plasma spray process are its relatively high cost, complexity of the process, slow deposition rate and large amounts of electricity required.
Wire-Arc Spray Process:
In the Wire-Arc Spray Process a pair of electrically conductive wires are melted by means of an electric arc. The molten material is atomized by compressed air and propelled towards the substrate surface. This is one of the most efficient methods of producing thick coatings. “In the two-wire arc process, two insulated metallic wire electrodes are continuously fed to an arc point where a continuously flowing gas stream is used to atomize and spray the molten electrode material in the arc. Some configurations utilize a single feed wire and non-consumable electrode” (U.S. Pat. No. 6,001,426).
Electric arc spray coatings are normally denser and stronger than their equivalent combustion spray coatings. Low running costs, high spray rates and efficiency make it a good tool for spraying large areas and high production rates.
Disadvantages of the electric arc spray process are that only electrically conductive wires can be sprayed and if the substrate requires preheating, a separate heating source is needed.
High Velocity Oxygen Fuel (HVOF) Thermal Spray Process:
The HVOF thermal spray process is basically the same as the Combustion Powder Spray Process (LVOF) except that this process has been developed to produce extremely high spray velocity. There are a number of HVOF guns which use different methods to achieve high velocity spraying. One method is basically a high pressure water cooled HVOF combustion chamber and a long nozzle. Fuel (kerosene, acetylene, propylene and hydrogen) and oxygen are fed into the chamber the chamber, combustion produces a hot high pressure flame which is forced down a nozzle increasing in velocity. Powder may be fed axially into the HVOF combustion chamber under high pressure or fed through the size of a laval type nozzle where the pressure is lower.
The coatings produced by HVOF are similar to those produced by the detonation process. HVOF coatings are very dense, strong and show low residual tensile stress or in some cases compressive stress, which enable very much thicker coating to be applied than previously possible with other processes.
Detonation Thermal Spraying Process:
The Detonation gun basically consists of a long water cooled barrel with inlet valves for gases and powder. Oxygen and fuel (acetylene most common) is fed into the barrel along with a charge of powder. A spark is used to ignite the gas mixture and the resulting detonation heats and accelerates the powder to supersonic velocity down the barrel. A pulse of nitrogen is used to purge the barrel after each detonation. This process is repeated many times per second. The high kinetic energy of the not powder particles on impact with the substrate result in a build up of a very dense and strong coating.
For reference a copy of Table 3 from U.S. Pat. No. 6,001,426 is presented which compares existing thermal spray technologies.
TABLE 3Comparison of thermal spray technologies.Flame powder: Powder feedstock, aspirated into the oxygen/fuel-gasflame, is melted and carried by the flame onto the workpiece.Particle velocity is relatively low, and bond strength of depositsis low. Porosity is high and cohesive strength is low. Spray ratesare usually in the 0.5 to 9 kg/h (1 to 20 lb/h) range. Surfacetemperatures can run quite high.Flame wire: In flame wire spraying, the only function of the flameis to melt the material. A stream of air then disintegrates themolten material and propels it onto the workpiece. Spray rates formaterials such as stainless steel are in the range of 0.5 to 9 kg/h(1 to 20 lb/h). Substrate temperatures are from 95 to 205° C.(200 to 400° F.) because of the excess energy input required forflame melting.Wire arc: Two consumable wire electrodes are fed into the gun,where they meet and form an arc in an atomizing air stream. The airflowing across the arc/wire zone strips off the molten metal,forming a high-velocity spray stream. The process is energyefficient: all input energy is used to melt the metal. Spray rateis about 2.3 kg/h/kW (5 lb/h/kW). Substrate temperature can be lowbecause energy input per pound of metal is only about one-eighththat of other spray methods.Conventional plasma: Conventional plasma spraying providesfree-plasma temperatures in the powder heating region of 5500° C.(10,000° F.) with argon plasma, and 4400° C. (8000° F.) withnitrogen plasma - above the melting point of any known material. Togenerate the plasma, an inert gas is superheated by passing itthrough a dc arc. Powder feedstock is introduced and is carried tothe workpiece by the plasma stream. Provisions for cooling orregulation of the spray rate may be required to maintain substratetemperatures in the 95 to 205° C. (200 to 400° F.) range.Typical spray rate is 0.1 kg/h/kW (0.2 lb/h/kW).Detonation gun: Suspended powder is fed into a 1 m (3 ft) long tubealong with oxygen and fuel gas. A spark ignites the mixture andproduces a controlled explosion. The high temperatures and pressures(1 MPa, 150 psi) that are generated blast the particles out of theend of the tube toward the substrate.High-Velocity OxyFuel: In HVOF spraying, a fuel gas and oxygen areused to create a combustion flame at 2500 to 3100° C. (4500 to5600° F.). The combustion takes place at very high chamberpressure (150 psi), exiting through a small-diameter barrel toproduce a supersonic gas stream and very high particle velocities.The process results in extremely dense, well-bonded coatings, makingit attractive for many corrosion-resistant applications. Eitherpowder or wire feedstock can be sprayed, at typical rates of 2.3 to14 kg/h (5 to 30 lb/h).High-energy plasma: The high-energy plasma process providessignificantly higher gas enthalpies and temperatures especially inthe powder heating region, due to a more stable, longer arc andhigher power density in the anode nozzle. The added power (two tothree times that of conventional plasma) and gas flow (twice ashigh) provide larger, higher temperature powder injection region andreduced air entrainment. All this leads to improved powder melting,few unmelts, and high particle impact velocity.Vacuum plasma: Vacuum plasma uses a conventional plasma torch in achamber at pressures in the range of 10 to 15 kPa (0.1 to 0.5 atm).At low pressures the plasma is larger in diameter, longer, and hasa higher velocity. The absence of oxygen and the ability to operatewith higher substrate temperatures produces denser, more adherentcoatings having much lower oxide contents.